QoS campus design / telephony / avaya / diffserv

I have implemented a QoS design for a customer with about 5000 nodes per campus. The design is relatively straigt through. Once you have decided with classes you want to implement, you have to configure the different devices. There has been an access, distribution and core layer. It’s the best to mark as close to the apllications as possible. So at the access layer we had 3750, 4000/4500. The 3750 supports srr and for the 4000 it’s dependend on the module. But for the 4000/45000 you don’t have input queues. As forexample the 6500 it depends all also on the module. You have to find out what kind of hardware queues there are. It’s probable a notation like 1P3Q8T, what means something like, 1 priority queue, 3 normal queues and 8 thresholds per queue. Sometime you can find also the notation 4Q8T/1P3Q8T, that means the one priority queue is able to be a normal or a prio queue.

If you have measurements about the actual network traffic, you can distribute the traffic on the different queues and thresholds. If not, it’s maybe good enough to have a priority queue and to roughly estimate the other queues, but maybe not go to deep into changing the default thresholds. Later it might be necessary. If you don’t have enough queues left and you want to keep up you queuing schema.

There is a good design guide for enterprise campus QoS implementation and i suggest taking this as a good starting point in you QoS campus design. It coves all the different catalyst types and also gives some suggestions about the ECN/DBL (dynamic buffer limiting) marking espacially for the 4000/4500’er catalyst.

This could be a good feature when both stations (server/client) support the ECN flag. I read about the xp/vista does it not have enabled by default. But it’s only available at the 4000/4500’er.

Make sure you use hardware queueing and not queuing in software. This will save you from having problems with cpu overload. As long queueing is done in hardware, you will be on the safe side.

Avaya does not have recomandations about qos implementations on cisco hardware. Phones can be configures like setting voice bearer traffic to COS 5 and Signaling traffic to COS 3. You can overwrite the data port with the PC connected to COS 0. This would be a relativ straight forwarded setup.

QoS is a quite complex task. It’s necessary to develop and administrate the current needs constantly.




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